Mobile phones and other telecommunication devices are prevalent in society. Users can communicate using telecommunication devices in various ways, such as, placing the telecommunication device in close proximity to the user's ear; using a speakerphone capability; or using a wearable headset. Each of these ways to communicate has some disadvantage. For example, while not clearly understood, some studies have shown potential health risks to using a mobile phone in close proximity to the user's head. Wearing a headset can be inconvenient, uncomfortable, aesthetically unpleasing, and inefficient when receiving and making calls. For example, putting on an earpiece when receiving a call is inconvenient as it takes time and effort. Speakerphones use conventional loudspeaker systems. Accordingly, there can be privacy issues and acoustic feedback problems for the user of speakerphones.
The use of highly directional sound transmission technology can be best described when taking a light bulb and torch light/spot light as examples. In the case of light bulbs, the light will generally disperse equally in all directions. The spotlight will, on the other hand, project a more focused beam of light. A similar observation can be made when considering conventional loudspeaker systems used in homes vs. highly directional loudspeaker systems. The former has poor directionality and the latter high directionality. Furthermore, generally speaking, low frequency audio signals have poor directivity and can be heard from “anywhere” which explains positioning strategies of subwoofers in any given room/space—the sound will travel in all directions and it will not matter greatly where one places the subwoofer. High frequencies audio signals, however, are more directional and thus have to be placed strategically so as to ensure that the sound reaches the listener as directly as possible. For example, turning the loudspeaker with tweeters and mid-range loudspeaker cones away from the acoustic sweet spot of hearing will result in poor sound for a listener. However, due to conventional loudspeaker system technologies, sound will still be heard with relative clarity.
Creating a sound projection system that is highly directional is generally achieved through two means. One is acoustic beamforming and the other is through ultrasound modulation. Acoustic beamforming has the advantage of employing conventional loudspeakers which directly generate sounds in the audible range. Acoustic beamforming typically works by employing an array of loudspeakers that are amplified separately with individual sound modification and signal processing blocks that filter the signal using specific filter coefficients. The net result of the individual filters and individual amplifiers affect the resulting constructive and deconstructive interferences contributed by each audio signal. The phase delays and shaping of the frequency responses of each audio channel results in fine directional control, which creates highly directional sound beams. In the case of the ultrasound system, the sound modification part is very different. As shown in FIG. 3, the desired sound (e.g., music) acts as a modulator for a high frequency (ultrasound) carrier frequency. The carrier frequency is typically at 40 kHz or above, which is above the hearing range of humans and thus cannot be heard directly. However, the resulting modulated ultrasound interacts with air in a non-linear way, which in turn produces an audible signal. The audible artifacts result when sound waves interact in non-linear mediums such as fluids and air, a phenomenon known as “nonlinear interaction of sound waves” or the “scattering of sound by sound” governed by equations as described in 1 P. J. Westervelt, “Parametric Acoustic Array,” J. Acoust. SOc. Am. 35, 535-537 0963 and also 1 Yoneyama, Masahide; Jun Ichiroh, Fujimoto (1983). “The audio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker design”. Journal of the Acoustical Society of America 73 (5): 1532-1536. The result is a highly directive sound beam that can be used to direct sound to very specific spots—a person in a crowd, for example, where others next to this person will hear nothing. The audible sounds are created by a concept referred to as difference frequencies in conjunction with non-linear interaction with the air functioning as the loudspeaker itself for the audible sound, which are typically finely tuned to the carrier's center frequency. Prior difference frequency utilizing systems were impractical in creating high bandwidth audible acoustic signals.